1. Field of the Invention
The present invention relates to an image forming apparatus, including a plurality of printing assemblies arranged in series, intended to make a multi-color record.
2. Description of the Related Art
The image forming apparatus of the electrostatic recording type comprises a photosensitive body (photosensitive drum), a charger, an optical head for forming an electrostatic latent image on the photosensitive body, a developing device for developing the electrostatic latent image into a toner image, and a transfer element (transfer roller) for transferring the toner image onto paper. The charger, the optical head, the developing device and the transfer element are arranged to surround the photosensitive body to form a charged toner image while the photosensitive body rotates one revolution. The charged toner image is transferred from the photosensitive body onto paper by a transfer element, and the paper is then sent to a fixing device and is discharged.
The photosensitive body, the charger, the optical head, the developing device and the transfer element constitute a printing assembly. The image forming apparatus capable of making a multi-color record comprises a plurality of printing assemblies each including a photosensitive body, a charger, an optical head, a developing device and a transfer element. When, for example, developing agents of four colors, i.e., yellow, magenta, cyan and black are to be used, the image forming apparatus is equipped with four printing assemblies containing respective developing agents. The image forming apparatus of the tandem type in which a plurality of printing assemblies are arranged in series, is particularly effective in executing printing at high speed.
The paper is conveyed through the plurality of printing assemblies arranged in series, and a fixing device arranged after the assemblies, by a paper conveyer belt. The toner images formed by the printing assemblies of the respective colors are superposed on the paper to form a color image. In order to bring the printing positions into agreement, the printing assemblies print positioning marks on the paper conveyer belt, and an optical sensor reads the marks printed on the paper conveyer belt. Based on the marks thus read out, the printing positions and the printing timings of the plurality of printing assemblies can be adjusted.
The image forming apparatus is further equipped with a top cover which is rotatably mounted on the frame. Upon opening the top cover, the interior of the frame becomes accessible. That is, upon opening the top cover, the constituent members of the print assemblies arranged inside the frame can be repaired or replaced. In particular, the toner must be replenished as it decreases in the developing device. It is desired that the toner can be replenished with ease. Usually, the top cover is locked to the frame by a lock member, and is opened by hand after the locking of the lock member is released.
In the image forming apparatus of the electrostatic recording type, one print assembly includes a photosensitive body, a charger, an optical head, a developing device and a transfer element. The photosensitive body, the charger and the developing device are detachably mounted to the frame as a developing unit. The optical head is mounted to the top cover. When the top cover is opened, therefore, the optical head is lifted together with the top cover. Thus, the user is allowed to access the developing device and so on, and the manufacturer is allowed to maintain the optical heads.
In the image forming apparatus equipped with a plurality of printing assemblies, a plurality of optical heads are mounted to the top cover, and the developing units are detachably mounted to the frame. Moreover, the top cover includes a control board for controlling the optical heads, sheet metal for shielding the control board, and a base frame for mounting the optical heads and the control board. Therefore, the top cover is very heavy. Besides, the image forming apparatus has been designed to be capable of effecting the printing even on large papers and, hence, employs large constituent members, causing the top cover to become even heavier. If the top cover becomes heavier, it is difficult for the operator to lift the top cover. Besides, many constituent members must be efficiently arranged in the limited space in the interior of the apparatus.
A laser beam must not be allowed to leak to the exterior when the top cover is opened. A laser beam which leaks in unexpected directions when the top cover is opened may enter the eyes of the operator. This is not desirable.